1. Field
Embodiments of the invention relate to the field of proximity sensors; and more specifically, to proximity sensor arrangements having a protective layer above the sensor to improve resistance to smudges on the protective layer and to reduce crosstalk between the emitter and detector due to the protective layer.
2. Background
Proximity sensors are used to sense hover events in a wide variety of devices including laptop computers, tablet computers, and smart phones. Hover events are no touch, close proximity positioning of parts of the user's body or other objects (e.g., a stylus held by the user), near or on an external surface of the device. It will be appreciated that “no touch” indicates that the sensor is not touched and that the external surface of the device may or may not be touched.
Typically such proximity sensors are designed to detect an external object that is located outside the near field detection capability of a touch sensor (e.g., those used in a typical touch screen display such as found in an iPhone™ device by Apple Inc.). In one instance, the proximity sensor includes an infrared emitter and a counterpart infrared detector that are controlled and sampled by proximity sensor circuitry integrated in the housing of the mobile device. Emitted infrared radiation may be scattered and/or directed toward the detector by the external object. Infrared radiation is detected and analyzed to infer that an external object is (or is not) close to the exterior surface. Because the detector receives emitted radiation that is reflected by the external object, such sensors may be referred to as reflective proximity sensors.
In the case of handheld mobile communications devices, the sensor may be located near an acoustic aperture for an earpiece speaker (receiver) of a mobile communications handset. This arrangement is used to determine when the handset is being held close to the user's ear, as opposed to away from the ear. When the proximity sensor indicates that the external object, in this case, the user's ear or head, is sufficiently close, then a predetermined action is taken, including, for example, turning off or disabling a touch screen display that is on the same external face of the housing as the acoustic aperture. This, of course, is designed to avoid unintended touch events caused by the user's cheek, while the handset is held close to the user's ear during a call.
The external surface of the device typically provides a protective layer above the proximity sensor and its associated electronics. The protective layer is made of materials that allow electromagnetic radiation of the wavelengths detected by the detector to pass through the protective layer. However, the protective layer itself will scatter and/or direct emitted infrared radiation toward the detector and interfere with detection of an external object. Smudges and dirt on the exterior surface can aggravate this interference.
It would be desirable to provide an exterior surface that provides a protective layer over a reflective proximity sensor in a way that reduces the effect of reflections and scattering from the protective layer and provides improved smudge resistance and reduced crosstalk between the emitter and the detector in the absence of an external object.